The control of magnetic tape motion and position in reel-to-reel tape drives is described in detail in U.S. Pat. Nos. 4,015,799, 4,125,881, and 5,576,905, all assigned to the assignee of this application and incorporated herein by reference in their entireties.
U.S. Pat. No. 4,015,799 relates to the use of the finely graduated, that is, a fine line tachometer on an idler roller engaging a magnetic tape to measure the amount of tape being advanced during a complete revolution of each tape reel shaft in a reel-to-reel tape drive system. The amount of tape advanced is converted to the radius of each reel once each revolution of the reel. Reel radius is then used to determine drive currents for each reel motor so as to provide a precise control of tape position and motion.
U.S. Pat. No. 4,125,881 describes a reel-to-reel tape drive in which magnetic tape is moved from one reel to the second reel, passing a read/write head mounted between the reels. A fine-line tachometer is mounted on one reel shaft to provide a fine-line tachometer reading in the form a number of pulses per revolution. A second tachometer on the second reel shaft provides a single pulse per revolution of the second reel. The single pulse is used to gate the counting of fine line tachometer pulses for each revolution of the second reel. A servo algorithm uses the gated-per revolution fine-line tachometer count to determine the real radii based upon the actual length and thickness of the magnetic tape whose position and motion the servo system controls. Motor acceleration currents of a magnitude corresponding to the real radii are generated to drive the reel motors.
Both of these incorporated patents are concerned with unidirectional tape drives in which magnetic tape is written and read in one direction. No recording occurs during movement of the tape in the opposite direction, which is used only for rewinding and repositioning the tape. However, in a bi-directional tape drive in which magnetic tape can be recorded in either direction the tape servo algorithm of these patents cannot accurately determine the radius of the tape reel and position of the data on the tape when the direction of tape writing is reversed. This problem was solved in the "905" patent.
U.S. Pat. No. 5,576,905 provided a compound solution to the problems of the first two patents by placing a fine-line tachometer on each drive motor for each reel of a reversible reel-to-reel tape drive. Each tachometer can include a single index line. In response to a signal conditioned to indicate the direction of motion for writing the tape, a fine-line output is selected from one of the tachometers fixed to the reel which is supplying the tape. When the direction in which data is being recorded or read is reversed, the roles of the tape reels reverse. Consequently, the source of the fine-line tachometer signal is switched to the tachometer on the motor driving the reel which is now supplying the tape. The provision of an index line on the tachometer on the motor which drives the reel solves the problem of positioning the reel which receives the tape leader block of the start of the tape. Once the threading notch is placed in the threading position, the tachometer is fixed to the shaft of the motor with its index mark at a known location. This provides a known correspondence between the index mark and the threading location in order to enable a threading servo to position the reel during all subsequent threading operations.
With the use of a bi-directional reel-to-reel tape drive, the servo control becomes very important because air could become entrapped on either reel and therefore the fine-line tachometer pulses now generated from the take-up reel would not correspond as accurately with the tape radius and tape position on the take-up reel. This is especially important since multiple data records on the tape are separated by inter-block gaps. The inter-block gaps (IBGs) are generated by timing the interval traveled between the records. A well controlled IBG has its size determined by the tape speed and the time interval. In order to maximize tape cartridge capacity, the size of the IBG is minimized.
When the writing process stops due to an interruption of data available from a host system or a write drive buffer, the tape drive must stop the tape and await the next write operation. Because of the very short length of the IBG and the relatively long stop and start distance required for the tape drive to accelerate, the tape drive motion servo system executes a "back hitch" in which tape motion is slowed following writing of the IBG, stopped, and then reversed back to a point where the read/write head precedes the location of the last written data. When the writing process begins again, the tape is accelerated from its stopped position up to a constant write velocity at which time the last data record and the IBG immediately following it has passed the read/write head and the next record is written.
In executing the back hitch operation, the position of the last written data recorded on the tape relative to the read/write head is controlled by the tape motion servo system by using the output of the fine-line tachometer and by measuring timing between the end of the last written data and a particular fine-line tachometer pulse. To start the back hitch, the data channel issues a synchronizing signal to the tape motion servo system indicating the end of the last data record. The tape motion servo system measures and stores the time lapse between the synchronizing signal and the next fine-line tachometer pulse which occurs. This pulse then becomes a position reference pulse. This time is subtracted from the desired IBG transit time to produce a time reference or partial IBG time for use in resynchronizing the recording channel circuits to the last data recorded on the tape. The fine-line tachometer pulses are counted for the purpose of locating the position reference pulse after the back hitch motion has been executed. When the position reference pulse is located, a write start point is achieved, and the tape motion servo system times out the remaining partial IBG time, issuing a resynchronization signal to the data channel when the time out is completed. The resynchronization signal thus occurs at the end of a nominal IBG distance from the previously written data record, and a new data record is appended.
The accuracy of the process of resynchronization during the back hitch operation is limited by the integrity of the fine-line tachometer pulses. In particular, the correspondence between the fine-line tachometer pulses and the position of the data on the tape relative to the read/write head is dependent on the radius of the tape stack. The tachometer pulses provide a measurement of the angular position of the reel which corresponds by radius to linear position of the tape. On the take-up reel, air entrapment increases the apparent radius of the tape stack, thereby compromising the integrity of the correspondence between the stack of tape on the reel and the reel hub.
The tape slack leads to tape mispositioning and tape damage. The tape mispositioning created either chopped IBG blocks or slivers of data. Manifestly, there is a need in a reversible reel-to-reel tape drive for solutions to the air entrapment problem and to the detection of servo anomalies that can cause these problems.